[BUG] perf stat: corrupts memory when using PMU cpumask

From: Stephane Eranian
Date: Fri Jan 17 2014 - 04:00:37 EST


I have been debugging a NULL pointer issue with perf stat unit/scale code
and in the process I ran into what appeared like a double-free issue reported
by glibc. It took me a while to realize that it was because of memory corruption
caused by a recent change in how evsel are freed.

My test case is simple. I used RAPL but I think any event with a suggested
cpumask in /sys/devices/XXX/cpumask will do:

# perf stat -a -e power/energy-cores/ ls

The issue boils down to the fact that evsels have their file descriptors closed
twice nowadays. Once in __run_per_stat() via perf_evsel__close_fd() and
twice in perf_evlist__close().

Now, calling close() twice is okay. However the fd is then set to -1.
That's still okay with close(). The problem is elsewhere.

It comes from the ncpus argument passed to perf_evsel__close(). It is
DIFFERENT between the evsel and the evlist when cpumask are used.

Take my case, 8 CPUs machine but a 1 CPU cpumask. The evsel allocates
the xyarray for 1 CPU 1 thread. The fd are first close with 1 CPU, 1 thread.
But then evlist_close() comes in and STILL thinks the events were using
8 CPUs, 1 thread and thus a xyarray of that size. And this causes writes
to entries that are beyond the xyarray when the fds are set to -1, thereby
causing memory corruption which I was lucky to catch via glibc.

First, why are we closing the descriptors twice?

Second, I have a fix that seems to work for me. It uses the evsel->cpus
if evsel->cpus exists, otherwise it defaults to evtlist->cpus. Looks like
a reasonable thing to do to me, but is it? I would rather avoid the double
close altogether.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/